1. Forces and the Laws of Motion
Chapter 4
Section 1

2. Changes in Motion : FORCE
Force is the cause of an acceleration or the change in an object’s velocity
Force can cause objects to
Start Moving
Stop Moving
Change Direction
Notice that each of these cases involves a change in velocity (this is acceleration).

3. Changes in Motion: FORCE

4. The SI unit of Force is the Newton named after Sir Isaac Newton.
The symbol for Newton is N.
Force is measured in Newton's.
Force is a Vector Quantity.
Force is equal to Mass times Acceleration.
1 N is equal to 1 kg x 1 m/s2

5. Forces can act through contact or at a distance.
Contact Force – force from the physical contact of two objects
Ex: catch a ball, its motion stops
Pull on a spring, it stretches
Field Force – does not involve physical contact between two objects.
Ex: the force of gravity
Electrical charges attraction/repulsion

6. Free-body Diagrams
Free – body diagrams isolate one object and the forces acting on it.
Shows only the forces acting on the object of interest.
All forces are assumed to act on a single point at the center of the object.

7. Section Review pg. 128
Work on problems 1-6

8. Newton’s First Law
Chapter 4 Section 4-2

9. Newton’s First Law of Motion
Also referred to as the Law of Inertia.
Inertia is the tendency of an object to maintain its state of motion.
Mass is a measurement of inertia.
States that an object at rest stays at rest and an object in motion stays in motion with constant velocity (constant speed in a straight line) unless the object experiences a net external force.
When the net external force of an object is zero, its acceleration is zero.

10. Acceleration is determined by net external force.
The Net External Force is the vector sum of all the forces acting on an object.
We find X components and Y components.
Net External Force is the Resultant Force.
Objects that are at rest or moving with constant velocity are said to be in equilibrium.
Equilibrium means there is no change in a body’s motion.

11. Let’s Practice…
Net External Force – Practice 4A
Pg. 133

12. Newton’s Second and Third Laws
Chapter 4 Section 3
Newton’s Second and Third Laws

13. Newton’s 2nd Law relates force, mass, and acceleration.
Net External Force = Mass X Acceleration
In equation form we have : ∑F = ma
∑F represents the vector sum of all external forces acting on an object.
States : Acceleration is produced when a Force acts on a Mass. The greater the mass of the object you are trying to accelerate, the greater the amount of force needed to do so.

14. Overview of Net External Force

15. Overview of Net External Force

16. Check for Understanding
What acceleration will result when a 12-N net force is applied to a 3-kg object?
Answer:
∑F = ma → 12N = 3kg x a
12N / 3kg = a
4 m/s2 = a

17. Try this one on your own…
A net force of 16 N causes a mass to accelerate at the rate of 5 m/s2. Determine the mass.
Answer:
∑F = ma → 16N = m x 5 m/s2
16N / 5m/s2 = m
3.2 kg = m

18. Sample 4B
Pg. 137

19. Newton’s Third Law Forces always exist in pairs.
For every action, there is an equal and opposite reaction.
Action =Ghost’s hand’s force exerted on wall.
Reaction = Wall’s force exerted on Ghost’s hand.

20. Check for Understanding…
If a book is on a table what are the action and reaction forces involved?
Answer:
Action : Force the book exerts on table.
Reaction : Force the table exerts on the book.

21. Newton’s 3rd Law Examples